The process of bubble formation in the hot isostatic pressing treated, doped molybdenum wire
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I.
INTRODUCTION
IN order
to increase recrystallization temperature, molybdenum is usually doped with K and Si in the form of oxides prior to sintering as tungsten is doped with A1, K, and Si (i.e., AKS-doped tungsten). Doped Mo wire that is annealed at temperatures above the recrystallization temperature shows elongated and interlocked grains a l o n g t h e w i r e axis. [11 S u c h a unique morphology of grains is considered to be formed by the bubble arrays produced by volatilization of dopants . [:] During the last two decades, many reports have been published on the formation of bubbles in AKS-doped tungsten. Most of them have dealt with the bubble formation mechanism, showing evidence that volatilization of K plays a dominant role in the bubble formation, I3-71 as well as the growing mechanism of the K-filled void. t8,91 There is, however, little information concerning the real substance of the entrapped materials in bubbles except the work of Snow, I71 who detected K in formed bubbles by means of selected area diffraction, using thin foils of doped, annealed tungsten wire. The present study was, therefore, undertaken in an attempt to visualize solidified entrapped materials present in the bubbles of doped Mo wire that was given a hot isostatic pressing (HIP) treatment and to analyze it by using energy dispersive X-ray (EDX) spectroscopy to obtain a better understanding of the volatilization process in the doped materials. II.
EXPERIMENTAL
Both 0.29 wt pct K and 0.2 wt pct Si were added to M o O 3 powder as dopants, utilizing a K2Si205 aqueous
solution. The doped powder was reduced under hydrogen atmosphere, then was compacted into a rectangular
JU CHOI, Director, and JAE-HOON LEE, Investigator, are with the Division of Metals and the Special Alloy Research Laboratory, respectively, Korea Institute of Science and Technology (KIST), P.O. Box 131, Cheongryang, Seoul, Korea. IN-HYUNG MOON, Professor, is with the Department of Materials Science, Han Yang University, Seoul, Korea. HYUNG SUP CHOI is a member of the National Academy of Sciences, Republic of Korea, Seoul, Korea. Manuscript submitted September 22, 1988. METALLURGICAL TRANSACTIONS A
bar; this was followed by sintering in H2 atmosphere at 2200 ~ for 30 minutes by direct current heating. This yielded a sintered density of about 90 pct of the theoretical one. The sintered rectangular bars of dimensions 85 mm x 85 mm • 300 mm were swaged and drawn to 1-mm-diameter wire, resulting in about a 98 pct reduction of the cross-sectional area. A part of the drawn wires was then isochronally annealed in the temperature range of 1000 ~ to 2000 ~ at intervals of 200 ~ under H2 atmosphere. Since the specimens annealed at 2000 ~ for 30 minutes had larger bubbles, the remainder of the as-drawn wires was hot isostatically pressed at 2000 ~ with 190 MPa for 30 minutes. Structural features of entrapped materials in bubbles that were found on the longitudinally fractured surface of hot isostatically pressed specimens were examined metallographically and constitution
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